[AMPS] Grid dissipation
Fri, 8 Aug 97 08:41:58 -0800
>> From: Rich Measures <email@example.com>
>> Date: Fri, 8 Aug 97 07:24:35 +0000
>> - There appears to be a lot of technoblather floating around about the
>> extreme delicacy of the 8877's grid. However, at the maximum ratings of
>> 250MHz, 1A, and 4000V, over 40 rms amperes of RF current flows through
>> the grid to chassis ground/common.
>You really should not give false advice like this to people trying to
>learn. Twisted statements like you made above ...
Taking a closer look at the allegations of false and twisted:
- The maximum ratings are from the (revised 5-15-77) Eimac Tech. Data
sheets for the 8877: 250MHz/1A/4000V. According to the characteristic
curves in these Tech. Data sheets, the minimum anode/plate to (grounded)
grid V is roughly 300V. With a 4000V anode supply, the resultant AC
anode voltage is 4000 minus 300V = 3700V peak, or 2600Vrms.
- According Eimac, the 8877 has about 10pF of C between the anode and the
grid. At 250MHz, 10 pF has 64 ohms of capacitive reactance. The AC
current through the grid = 2600V rms/64 ohms = 40.6A rms. ....... Sure,
40A is a fair amount of current. However, the 8877 has over 100
rectangular grid wires, so the current is only about 400mA per
gold-plated grid wire. Since gold is a fairly good conductor and gold
does not begin to evaporate in a vacuum until its temp. exceeds 1800
degrees F, why should there be a problem?
R. L. Measures, 805-386-3734, AG6K
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